Today optical fibers are the preferred channel for the transmission of information with large bandwidth, whether that is audio, video, data or others. Compared to copper-based twisted pair or coax networks, optical fibers have a much higher bandwidth, much smaller attenuation per unit of length and less susceptibility to electromagnetic interference. As optical fiber networks grow more complex and expand into the access network, it becomes important to monitor the network performance. Existing solutions examine the performance of specific network services during network operation, mainly relying on bit error rate (BER) measurements, but the possibility to monitor the physical layer is not as widespread because of hitherto high implementation and operation costs. Nevertheless, a fast preventative and continuous diagnosis of the integrity of the fiber network and a quick identification of deteriorating link performance can be an important asset in all kinds of performance-critical optical fiber links and networks.
A system that allows round-the-clock monitoring of the fiber network status and provides knowledge of the fiber integrity can also be a strong driving force behind full-scale deployment of passive optical fiber access networks. In order to characterize the channel response of a passive optical access network compliant with BPON (ITU-T G.983.1), GPON (ITU-T G.984.2), EPON (IEEE 802.3), or other standards, it is necessary to perform measurements from both the optical network unit (ONU) and the optical line termination (OLT) side. This is necessary because it is very difficult to detect and distinguish reflections from behind the optical splitter when only measuring from the OLT side.
Generally, optical time domain reflectometry (OTDR) is the preferred method for defining the exact cause of a localized link deterioration of an optical fiber. Problems can result from fiber breaks, splice losses, and distributed losses due to fiber ageing. Other causes are possible as well. An OTDR system excites the fiber with an optical pulse, whose width is a trade-off between the distance resolution and measurement sensitivity.
Commercial off-the-shelf OTDR instruments are already deployed to monitor fibers from long-haul networks, where all optical fibers at the cabinet are tested by the same instrument and routed to the fiber under test with expensive optical switches. This architecture enables sharing the expensive monitoring equipment at the expense of costly additional optical components. It is difficult to apply this expensive and complex strategy to optical access networks, especially if one wants to measure from the ONU side. Current OTDR equipment is widely used but not suited for embedded use because of the invasive procedure. It requires data transmission to be stopped, the optical fiber link to be opened in order to enable the injection of OTDR pulses into the fiber, the detection and processing of the echo signals, and finally the restoration of the optical link and re-establishment of the data traffic.
An embedded OTDR system must meet three major criteria. First, the measurements should interfere as little as possible with the data traffic being transmitted over the optical fiber. Various techniques have been proposed to reduce or avoid interference, all suffering from the drawback that separate OTDR signals must be transmitted. A second requirement for embedded OTDR modules is that the system cost be low. The community of network providers adopts only cost-effective solutions to monitor all sections of the millions of kilometer of fiber that are operative worldwide. This is especially critical for PON access networks, where an OTDR unit is needed inside every ONU. A third requirement is that the embedded OTDR module should not have a negative impact on the link performance. Techniques using a separate wavelength, an optical splitter/combiner or data modulation decrease the link budget.
In patent application EP-1524781-A1 and U.S. Patent Publication No. 2005/0201761 the laser driver and laser diode present for data communication in the fiber endpoint are reused. This is a good approach, as the measurement shows the attenuation in a function of distance at the communication wavelength. The OTDR curve is dependent on the excitation wavelength and cannot be measured accurately by using a separate OTDR wavelength. This system also significantly reduces cost because no dedicated OTDR laser and driver are needed. Based on the fact that upstream and downstream communication operate in a single fiber and at the same wavelength, U.S. Patent Publication No. 2005/0201761 reuses the data receiver to measure optical echoes. This limits its use to a semi-duplex communication channel and excludes PON applications.
This problem is solved in EP-1524781-A1 by using a dedicated receiver and one extra optical component, namely an optical coupler. However, the price of this solution is considerably higher, and the optical coupler decreases the link optical power budget with its loss. This method also requires network traffic to be halted temporarily and the transmission of specific OTDR signals.
A similar approach is found in document EP 1624593-A1, which relates to a method and system for monitoring a passive optical distribution network. In this approach, a monitoring signal is sent through an optical fiber link. Parts of the reflected monitoring signal are received. These parts are subsequently used for comparing signal losses of the link and for deciding whether a failure has occurred.
Document EP 1632766-A1 presents a method for reflectometric testing by detecting the optical echoes without need for additional optical components, making it less expensive and therefore more suitable to embedded monitoring. The system, however, is still intrusive because it requires measurement windows during which the data transmission is halted to perform reflectometric tests.
In “The effect of reflected and backscattered live traffic on CWDM OTDR measurements” (Iannone et al., IEEE PTL, vol. 16, no. 7, pp. 1697-1699, 2004) a traffic monitoring method is disclosed using another wavelength than upstream/downstream (often 1650 nm). While this is non-intrusive, the costs are increased. Further, measurements on different wavelength do not necessarily perfectly reflect the link quality.
In “New technique for non-intrusive OTDR based on traffic data correlation” (Electronic Letters, Vol. 30, no. 17, pp. 1443-1444, Aug. 18, 1994), Biain et al. disclose a method for non-intrusive optical TDR. Correlation of the transmitted traffic data with the backscatter of the transmitted data allows the power attenuation of the optical link to be measured.
There is a need for a fiber monitoring system that solves the above-mentioned drawbacks and that is suitable for low-cost integration into, for example, an optical transmitter or even into a laser driver chip. Such a system can be used in PON networks as well as in almost any optical fiber network, provided that sufficiently long “idle” time windows are present or can be allocated during which echo signals can be acquired.